1. Field of the Invention
This invention relates generally to an electronic flash control circuit and, more particularly, to a control circuit for timing the duration of a flash of artificial illumination provided by an electronic flash.
2. Description of the Prior Art
Electronic flash devices of the type in which the duration of the flash of artificial illumination is automatically controlled are well known in the art. One such electronic flash device of this type is commonly known as a quench strobe and embodies a quench tube which when triggered into conduction rapidly discharges the remaining charge of a primary storage capacitor which charge would have otherwise been discharged through the light emitting flashtube. In this manner, the flash of artificial illumination may be rapidly quenched or terminated as is well known in the art. Other electronic flash devices in which the duration of the flash of artificial illumination may be controlled embody semiconductor switching devices such as thyristors in serial connection with respect to the light emitting flashtube. The thyristors may be switched into a nonconductive state to terminate the discharge of current through the flashtube thereby terminating the flash of artificial illumination. Regardless of how the flash of artificial illumination is actually terminated the aforementioned electronic flash devices derive a flash terminating control signal by first detecting the artificial and ambient scene light to provide an output signal corresponding to the intensity of the ambient and artificial scene light so detected. The output signal is thereafter integrated to a selected value to provide the flash termination control signal. In this manner a uniform amount of artificial scene light may be provided which corresponds to the selected value to which the output signal for the detected scene light is integrated to.
Under conditions of low or insignificant ambient scene light intensity where an electronic flash may be expected to be most commonly utilized to provide the major portion of the film exposing illumination, the aforementioned system operates quite satisfactorily since the ambient scene light intensity detected is only a negligible portion of the overall detected scene light. However, under conditions where the electronic flash is utilized to provide a fill-in flash of artificial illumination under conditions of relatively high ambient scene light intensity to brighten shadows, ambient scene light intensity is no longer a negligible proportion of the overall detected scene light and thus affects the timing of the flash of artificial illumination. As is readily apparent, the higher the ambient scene light intensity during the operation of the flash device, the less artificial scene light will be required to be detected for the integration circuit to reach its selected value and hence the electronic flash device can no longer be relied upon to provide a uniform amount of artificial scene illumination.
Therefore, it is a primary object of this invention to provide a control circuit for timing the duration of a flash of artificial illumination from an electronic flash in a manner ensuring that the electronic flash device provides a substantially uniform amount of illumination regardless of whether the flash is utilized in its ordinary manner to provide substantially all of the illumination required to properly expose the film or in its fill-in flash mode of operation wherein the electronic flash provides only a small proportion of the overall illumination required to expose the film.
It is a further object of this invention to provide a control circuit for timing the duration of a flash of artificial illumination from an electronic flash in a manner whereby the amount of artificial illumination may be maintained substantially uniform regardless of the intensity of the ambient scene light.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a mechanism and system possessing a construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.